The density is a measure of a mediums mass per unit of volume. The SI unit of density is the kilogram per cubic meter (kg/m3). As examples, at standard temperature and pressure condition, the density of water is 1000 kg/m3, of ethyl alcohol is 790 kg/m3, respectively.
Viscosity is a measure of the resistance of a fluid to deformation. Viscosity describes a fluid internal resistance to flow and may be thought of as a measure of fluid friction. The SI physical unit of dynamic viscosity is the Pascal second (Pa·s). The cgs physical unit for dynamic viscosity is the poise (P). It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). As example, water has a viscosity of 1.0 cP (at 20° C.).
It is known from patent document EP0282251, a transducer for measuring density or viscosity of fluid. The transducer comprises two parallel tines extending from a common yoke, forming a tuning fork adapted to be immersed in the fluid. The tines are excited to vibrate resonantly and in anti-phase by one or more piezoelectric exciting elements, which are housed in one or more cavities within the tines or the yoke. The vibrations are sensed by one or more similarly-housed piezoelectric sensing elements.
This transducer has a typical size of several centimeters requiring a bulky and expensive sensor housing. This leads to a highly intrusive measuring device preventing its use in small sample chambers or small pipes. Accuracy of such device is limited by pressure and temperature effects on the sensor materials requiring adding complementary pressure and temperature sensors for corrections. Operation at high pressure and high temperature is also very difficult to implement due to the large size and due to the piezoelectric transducer elements implementation inside the sensor housing. Further the sensor geometry does not allow to align the sensing element with the flow leading to large measurement perturbations at high fluid velocities.
The patent application EP05290502.3 filed on 4 Mar. 2005 discloses a miniaturized density and viscosity sensor. This sensor requires additional temperature and pressure sensor for correcting the effect of temperature and pressure variation on the frequency and quality factor impacting density and viscosity measurements. The membrane movement generates a large acoustic signal in the fluid that may induce perturbation signal by microphonic effect. The piezoelectric element may be submitted to high stress at high pressure. The excitation detection scheme is based on frequency sweep which is time consuming in the determination of density and viscosity.